In some cases, it is necessary to measure a propagation time of a sound wave from a speaker to a microphone in a space in which an acoustic system is installed. This corresponds to, for example, cases where a frequency characteristic of the acoustic system is measured at a listening position, and a signal having a frequency characteristic that varies with time is used as a sound source signal for measurement. In such cases, measurement with higher precision is sometimes achieved by taking in a signal from the microphone installed at the listening position after passing the signal through a filter that varies its frequency characteristic according to a time variation in the frequency characteristic of the sound source signal for measurement, rather than by directly taking in the signal from the microphone installed at the listening position. In this case, it becomes necessary to delay the variation in the frequency characteristic of the filter by time for which the sound wave propagates over a distance from the speaker to the listening position, instead of simultaneously progressing the variation in the frequency characteristic of the sound source signal for measurement and the variation in the frequency characteristic of the filter. For this purpose, it is necessary to measure the propagation time of the sound wave from the speaker to the microphone installed at the listening position.
Accordingly, there has been conventionally proposed a method of measuring a propagation time of a sound wave between a speaker and a microphone using a pulse (see for example, Japanese Laid-Open Patent Application Publication No. 2001-112100 (see page 3, FIGS. 1 and 2)). Specifically, a propagation time of a pulse sound which is output from the speaker and arrives at the microphone is measured.
Measurement using the pulse sound can be conducted with relatively higher precision unless it is affected by a noise. However, since the pulse sound has a small energy with respect to its amplitude, it is difficult for the microphone to receive the sound with a preferred S/N ratio. In this method, therefore, accurate measurement is not always conducted.
In order to improve this method, the applicant has made an attempt to measure a propagation time of a sound wave having a sweep signal as a sound source, as a signal having a relatively large energy with respect to its amplitude. Specifically, the sweep signal which is frequency-swept in a short time is input to a speaker, which outputs a sweep sound, which is received by a microphone. And, arrival time of the sound wave is measured for each frequency band.
If the sweep signal as the sound source signal is known, it is possible to know when a component in each frequency band is output from the speaker. In addition, it is possible to know arrival time of the component in each frequency band by band pass filtering the signal received by the microphone.
By finding an effective value of the signal in each frequency band received by the microphone for a fixed duration while slightly shifting a time starting point, a root-means square (RMS) value as a function of the time starting point may be found, and a time point at which the RMS value becomes maximum may be assumed to be the arrival time of the component in each frequency band. This enables more accurate measurement of a distance.
This method has advantages as follows: {circle around (1)} A frequency band with a higher level can be selected because of the use of a plurality of frequency bands. {circle around (2)} Interference from a noise is less because of the use of the band pass filter. {circle around (3)} The sweep signal is resistant to a noise because it has an energy larger than that of the pulse.
On the other hand, this method has disadvantages as described below. The response is slow because of the use of the band pass filter. A measurement value may be corrected in view of a known delay of a response time. But, if the response time of the band pass filter is larger than the propagation time of the sound wave between the speaker and the microphone, measurement precision is not ensured. While the signal is less affected by the noise as the frequency band of the band pass filter decreases, the response time of the band pass filter increases.
The response time of the band pass filter decreases as the frequency band of the band pass filter increases, but the signal is susceptible to the noise. Further, a frequency characteristic of an acoustic system in that frequency range may appear, which may cause a peak value of the signal in a frequency other than a target frequency to be detected. This may lead to inaccurate measurement.